1.1 Particles in the atom and atomic radius

An atom is mostly empty space. A very small, dense, positively charged nucleus contains protons and neutrons, with electrons arranged in shells around it.

The nucleus contains nearly all of an atom’s mass because protons and neutrons are much more massive than electrons. In a neutral atom, the positive charge of the protons is balanced by the negative charge of the electrons.

The number of protons identifies the element; changing the number of neutrons gives an isotope, while changing the number of electrons gives an ion. These changes do not move the nucleus from the centre of the atom.

Shells describe allowed electron energy levels or regions, not fixed miniature planetary orbits. Detailed electron configurations, orbital shapes and atomic-radius trends belong to the neighbouring objectives.

Atoms contain three main subatomic particles: protons and neutrons in the nucleus, and electrons in shells around the nucleus. The particles differ in relative charge, relative mass and location.

Protons have relative charge +1 and relative mass 1; neutrons have charge 0 and relative mass 1; electrons have charge −1 and a much smaller relative mass. Protons and neutrons are nuclear particles, whereas electrons occupy the space around the nucleus.

Because protons and neutrons are much more massive than electrons, nearly all atomic mass is concentrated in the nucleus. In a neutral atom, equal numbers of protons and electrons make the total charge zero.

Relative values are used to compare the particles; they are not exact masses. A change in electron number makes an ion, but changing the proton number changes the element. Detailed electron configurations belong to the neighbouring objectives.

The atomic number, Z, is the number of protons in the nucleus. It identifies the element because every atom of that element has the same proton number.

The mass number, A, is the total number of protons and neutrons in the nucleus. Therefore, neutron number = A − Z. Electrons are not included in the mass number because their mass is negligible relative to nuclear particles.

A nuclide can be written as ^A_ZX: A is the mass (nucleon) number at the upper left, Z is the atomic (proton) number at the lower left, and X is the element symbol. Read both numbers before identifying the particle counts.

Changing Z changes the element; changing A while keeping Z fixed changes the isotope. Do not confuse mass number for one nuclide with relative atomic mass, which is an average based on isotope abundances.

An atom is mostly empty space, but its mass and charge are not spread evenly. Nearly all the mass and all the positive charge are concentrated in a very small nucleus.

Protons and neutrons each have relative mass about 1, whereas an electron has a much smaller relative mass. This is why the nucleus contains nearly all the atom’s mass even though electrons occupy the surrounding region.

The nucleus is positive because it contains protons; the surrounding electrons contribute negative charge. In a neutral atom these charges balance overall, although the positive charge remains concentrated at the centre.

Do not describe an atom as a solid sphere or place electrons inside the nucleus. Mass concentration and charge concentration are related but distinct ideas; particle identities and electron arrangements are handled by neighbouring objectives.

In an electric field, a charged particle experiences a force toward the oppositely charged plate; a neutral particle is not deflected by this electric force. Comparing beams at the same speed reveals how charge and mass affect the deflection.

A proton has relative charge +1 and relative mass 1, so it bends toward the negative plate. An electron has charge −1 and a much smaller mass, so it bends toward the positive plate and is deflected more strongly. A neutron has zero charge and continues undeflected in the electric field.

The direction of deflection identifies the sign of charge, while the amount of bending depends on the force relative to the particle’s mass. The electron’s very small mass gives it a much larger acceleration than a proton in the same field.

The comparison assumes beams with the same speed and a stated electric-field arrangement. Do not infer particle identity from deflection alone without checking the field direction and the experimental conditions; magnetic-field details are not added beyond the supported objective evidence.

Read isotope notation ^A_ZX by taking A as the mass (nucleon) number and Z as the atomic (proton) number. X identifies the element symbol; the two numbers give the nuclear particle counts.

Proton number = Z. Neutron number = A − Z. The mass number counts only protons and neutrons, so it is the total number of nucleons in the nucleus.

For a neutral atom, electron number = Z. For an ion, adjust the electron count for the stated charge: a positive charge means electrons have been lost, while a negative charge means electrons have been gained.

Use a fixed sequence: read A and Z, calculate A − Z for neutrons, then use the charge only to adjust electrons. Keep the element identity tied to Z; changing electron number changes charge, not the element.

Atomic radius describes the size of an atom, while ionic radius describes the size after electrons have been gained or lost. Both are controlled by the attraction between the nucleus and the outer electron region.

Across a period, proton number increases while electrons are added to the same principal shell. Nuclear attraction therefore becomes stronger overall, so atomic radius generally decreases across the period.

Down a group, each step adds an occupied electron shell. The outer electrons are farther from the nucleus and more shielded by inner shells, so atomic radius generally increases down the group.

A positive ion is usually smaller than its atom because electron loss reduces the outer electron region; a negative ion is usually larger because added electrons increase electron–electron repulsion. Use the stated species and a consistent radius definition when comparing values.